The present invention relates to ink jet printing apparatus and is concerned, more particularly, with the maintenance of a printhead in such apparatus.
An ink jet printer of the so-called "drop-on-demand" type has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink may be contained in a plurality of channels and energy pulses are used to cause the droplets of ink to be expelled, as required, from orifices at the ends of the channels.
In a thermal ink jet printer, the energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As a vapor bubble grows in any one of the channels, ink bulges from the channel orifice until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel and towards the recording medium. The channel is then re-filled by capillary action, which in turn draws ink from a supply container. Operation of a thermal ink jet printer is described in, for example, U.S. Pat. No. 4,849,774.
One particular form of thermal ink jet printer is described in EPA 0,210,848. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information.
It has been recognized that there is a need to maintain the ink ejecting orifices of an ink jet printer, for example, by periodically cleaning the orifices when the printer is in use, and/or by capping the printhead when the printer is out of use or is idle for extended periods. The capping of the printhead is intended to prevent the ink in the printhead from drying out. There is also a need to prime a printhead before use, to ensure that the printhead channels are completely filled with ink and contain no contaminants or air bubbles. Maintenance and/or priming stations for the printheads of various types of ink jet printer are described in, for example, U.S. Pat. Nos. 4,855,764; 4,853,717 and 4,746,938 while the removal of gas from the ink reservoir of a printhead during printing is described in U.S. Pat. No. 4,679,059.
It has been found that the priming operation, which usually involves either forcing or drawing ink through the printhead, can leave drops of ink on the face of the printhead and that, ultimately, there is a build-up of ink residue on the printhead face. That residue can have a deleterious effect on print quality. It has also been found that paper fibers and other foreign material can collect on the printhead face while printing is in progress and, like the ink residue, can also have a deleterious effect on print quality. It has previously been proposed, in U.S. Pat. No. 4,853,717, that a printhead should be moved across a wiper blade at the end of a printing operation so that paper dust and other contaminants are scraped off the orifice plate before the printhead is capped. It has also been proposed, in U.S. Pat. No. 4,746,938, that an ink jet printer should be provided with a washing unit which, at the end of a printing operation, directs water at the face of the printhead to clean the latter before it is capped. In JP-A 62-9957, it is proposed that ink leaking from a printing nozzle should be absorbed by a piece of fibrous material in engagement with the nozzle.